The present invention relates generally to wireless communications. More particularly, the invention is directed to power control in spread spectrum communication systems.
The relative power used in each data signal transmitted in a spread spectrum communications systems require control in response to information transmitted by a base station and each remote unit. The primary reason for providing such control is to accommodate the many remote units that may be transmitting on the same frequency, such that all the transmitted signals are of the same approximate power so that none of the remote units are unusually disadvantaged. Unless the power being transmitted in the system is uniform among units, the signal quality may become unacceptable to those units at a lesser power, wherein a stronger transmitted signal will interferenee with a weaker signal. Thus, output power must be controlled to guarantee enough signal strength received at each unit to maintain good signal quality while minimizing the potential for interference.
Additionally, since a Code Division Multiple Access (CDMA), Wideband Code Division Multiple Access (WCDMA), and CDMA2000 wideband channel can be reused in every cell, self-interference caused by other users of the same call and interference caused by users in other cells represents a limiting factor to the capacity of the system. Moreover, the interference coming from the neighboring base stations may not fade with the signal from the active base station as would be the case for interference coming from the active base station. Due to fading and other channel impairments, maximum capacity is achieved when the signal-to-interference ratio (SIR) for every user is, on the average, at the minimum point needed to support xe2x80x9cacceptablexe2x80x9d channel performance. A remote unit in these situations may require additional signal power from the active base station to achieve adequate performance.
Communication systems are known to employ power control methods that control transmission energy of base stations and remote units. Power control in spread spectrum systems serves two main functions. First, because each remote unit""s signal in a spread spectrum system is typically transmitted in the same frequency, a majority of the noise (i.e., inversely proportional to bit energy per noise density, Eb/No, defined as the ratio of signal energy per information-bit to noise power spectral density) associated with a received signal can be attributed to other remote units"" transmissions. The magnitude of noise is directly related to the received signal power of each of the other remote units"" transmissions. Thus, it is beneficial for a remote unit to transmit at a low power level. Secondly, it is desirable to dynamically adjust the power of all remote units in such a way that transmissions are received by the base station with approximately the same power level. Similarly, the remote unit can request modification of the base station transmitter power to maintain a suitable level.
Dynamic power control of a mobile station transmitter includes two elements: open loop estimation of transmit power by the mobile station, and closed loop correction of the errors in this estimate by the base station. In open loop power control, each mobile station estimates the total received power on the assigned spread spectrum frequency channel. Based on this measurement and a correction supplied by the base station, the mobile station""s transmitted power is adjusted to match the estimated path loss, to arrive at the base station at a predetermined level. Closed loop corrections involve both the mobile station and the base station. After setting the initial level with the open loop estimate, the mobile station will begin its closed loop correction process, where the base station will send a power control bit to the mobile station in every transmission slot of a frame to tell the mobile to increase or decrease power. The step size of the power can change in 1 to 3 dB steps in each slot, depending on the communication system being used. In the same way, the mobile unit can send power control bits to the base station to request power modification.
For example, WCDMA base stations and mobile stations provide transmit power control (TPC) bits on the uplink and downlink dedicated physical control channels (UL DPCCH and DL DPCCH) in an effort to ensure that constant transmitter power is used between each. This system is described in 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 4), 3GPP TS25.214 v4.3.0 (2001-12), which is hereby incorporated by reference. Unfortunately, the time for a mobile unit to receive TPC information from a base station, measure power level, calculate the power change, and prepare to uplink is limited to 133 microseconds per slot. Moreover, the mobile station may also be required to calculate power amplifier gain corrections, power amplifier bias corrections, and the like during this same time period, result in a peak in processing requirements that may not be met during this short period. This time can be further shortened by multi-path delays, conversion delays, propagation delays, receive and transmit lineup delays, serial transfer delays and DSP interrupt, task switch and data transfer delays. In practice, these items can reduce the time left over for power control processing to less than 50 microseconds. Similarly, the base station can have a processing peak between the time when the mobile station sends TPC information and when the base station downlinks.
What is needed is a way to reduce computation complexity during these time periods to reduce peak processing. In would also be of benefit to provide time for all other necessary corrections. It would also be of benefit if the above improvement could be provided in a simple hardware implementation with minimal software requirements.